Buoyancy tensioning systems for offshore marine risers and methods of use

ABSTRACT

Devices and methods for the enhanced assembly and disassembly of offshore marine risers through the use of a combination of passive and active buoyancy elements in a marine riser. Methods for assembling passive buoyancy joints comprise providing disc handling devices capable of simultaneously manipulating a plurality of buoyancy discs capable of nesting and interlocking with one another. The combination of passive and active buoyancy elements along with a near surface disconnect package allow for quicker upper riser recovery especially in the event of an approaching tropical storm.

CROSS-REFERENCE TO RELATED APPLICATION

This nonprovisional patent application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/964,473, titled “Sea Hook Free Standing Riser (FSR),” which is hereby incorporated by reference.

BACKGROUND

The present invention generally relates to buoyancy tensioning systems and methods for offshore marine risers and more particularly, buoyancy tensioning systems for marine offshore riser systems that incorporate passive and active buoyancy elements and corresponding methods of use.

Offshore drilling for hydrocarbons typically involves an offshore drilling rig that connected to a wellhead via a marine riser. FIG. 1A shows an example of a conventional offshore drilling rig 1 connected to a wellhead 3 by a riser 2. Typically, riser 2 is comprised of coupled joints of pipe, each joint of pipe typically around 50-90 feet in length. Riser 2 often extends several thousands of feet to reach the seabed or ocean floor to wellhead 3 from offshore rig 1 (e.g. typically riser strings often extend in excess of 5,000 feet and sometimes extend up to 12,000 feet or more in extremely deepwater wells). In this configuration, offshore rig 1 provides the necessary tension on riser 2 to prevent riser 2 from collapsing or otherwise “falling over” under its own weight.

It is often desirable that a riser and correspondingly elements of riser, are negatively buoyant (i.e. sinks without assistance). Negatively buoyant riser elements are easier to install because they float down of their own volition, thus not needing an additional force to displace them to their installed position. Additionally, negative buoyancy acts to reduce the effects of currents and wave motion on the lateral motion of the riser. Nevertheless, a riser string that is too negatively buoyant will be too heavy for a rig to provide support, that is, the tension necessary to prevent the riser from collapsing under its own weight. Thus, it is desirable to counteract a large portion of the negative buoyancy to avoid a riser string that is “too heavy.” In some cases, operators will typically design riser elements as only slightly negatively buoyant. This slight negative buoyancy is usually accomplished by the incorporation of one of more passive buoyancy elements to somewhat offset the heavy weight of each riser element. For example, passive buoyancy elements 4 surround each joint of riser 2. Here each passive buoyancy element is shown as a syntactic foam element.

Conventional buoyancy elements such as those depicted in FIG. 1A are somewhat complicated to install, because several foam elements (whose typical length is 5-10 ft) must be installed on each and every joint of “buoyed” riser string. Further, because the passive buoyancy for riser 2 is produced by prefabricated foam elements, these elements must be fabricated long beforehand and cannot easily be resized for different riser configurations, needs, or conditions.

Occasionally, an operator may wish to move riser 2 to another wellhead location. With such a conventional system, riser 2 must be disconnected at wellhead 3, and then, retrieved joint-by-joint in a painstaking time-consuming process. Indeed, retrieving an entire riser often involves several days of intensive labor to successfully retrieve the entire length of an offshore marine riser. Where sufficient warning time is not available, operators are sometimes forced to abandon an entire riser string to allow departure of an offshore rig, resulting in significant capital loss. Thus, the conventional method of retrieving riser 2 is costly and time-consuming.

Alternatively, in the event of some emergency, such as an approaching hurricane, offshore rig 1 may need to be evacuated if moored, or moved out of the path of dangerous weather patterns to a safe location if dynamically-positioned. Again, in such a conventional system, entire riser string 2 would need to be retrieved joint-by-joint before being able to evacuate offshore rig 1. The conventional system of removing riser 2 to allow movement of rig 1 is costly and time-consuming. Frequently, it is difficult to predict the path of a hurricane or other weather emergencies. Some weather or meteorological/oceanographic emergencies such as loop or eddy currents or other fast-developing weather patterns may not afford an operator the necessary time to retrieve riser 2 by conventional methods to allow evacuation of or, in the case of non-moored, dynamically-positioned mobile offshore drilling units, evasive movement by rig 1. Additionally, a great deal of productive operational time is lost by the conventional systems because of the long duration required to retrieve riser 2. Moreover, cost and time is often expended for the retrieval of riser string that ultimately proves unnecessary given the changing path of a hurricane. In addition to the time and cost involved in retrieving the entire length of riser string, such a process involves additional safety risks, including the risks involved with manipulating the pipe being disassembled and handling the large volume of riser pipe at the rig.

Some operators have sought to address these disadvantages through the use of a “free-standing riser” design. An example of a free-standing riser design is depicted in FIG. 1B. As before, rig 1 communicates with wellhead 3 through riser 2. Here, however, near surface disconnect package (NSDP) 5 partitions riser 2 into upper riser section 8 and lower riser section 9. Upper riser section 8 extends from rig 2 through a shallow depth of about 500-1000 feet to a point where lateral water currents are minimal. NSDP 5 allows upper riser section 8 to be disconnected as desired from lower riser section 9. An active buoyancy element at the top of lower riser section 9, in this case, aircan 6, provides the buoyancy necessary to prevent lower riser section 9 (i.e. the free-standing riser) from collapsing or “falling-over” under its own weight. Active buoyancy elements operate by the introduction of a buoyant gas or other fluid that causes the active buoyancy element to be positively buoyant in water.

Such a design involves numerous disadvantages. Some of these disadvantages, which are enumerated by Chau Nguyen in Storm-Safe Deepwater Drilling (IADC/SPE 10338), include the necessity of modifying the rig to provide additional space to store, transport, and install the bulky aircan or aircans, which are integrally installed into the riser. Furthermore, additionally handling equipment on the rig is necessary to adequately deal with the bulky aircans. Compounding the problem of dealing with the volume of bulky aircans is the inability to install the integral aircans above the rotary table. Because of their large size, aircans must often be installed within the moonpool area below the rotary table. Dealing with this bulky equipment is problematic not only during initial installation of a riser string having aircans, but also during retrieval of a riser string. The large volume occupied on the rig by the bulky handling equipment and the bulky aircans pose not only serious safety risks but increase operational costs as well. Moreover, it slows down the process of inserting pipe joints into the riser.

Accordingly, it would be desirable to provide buoyancy tensioning systems that address one or more disadvantages of the prior art.

SUMMARY

The present invention generally relates to buoyancy tensioning systems and methods for offshore marine risers and more particularly, buoyancy tensioning systems for marine offshore riser systems that incorporate passive and active buoyancy elements and corresponding methods of use.

An example of a method of retrieving a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor comprises the steps of: providing a marine riser system comprising: a riser comprising an upper riser and a lower riser, a disconnect joint removably attaching the upper riser to the lower riser, and a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is non-integral to the lower riser and removably attached to the lower riser; introducing a buoyancy fluid into the at least one chamber of the second buoy; and disconnecting the upper riser from the lower riser at the disconnect joint.

An example of a buoyancy tensioning system for a marine riser comprises: a riser string defined by a first end and a second end; a disconnect joint disposed in said riser string; a blow out preventer stack attached to said second end of said riser string; a first buoy attached to said riser string adjacent to said disconnect joint; a landing ring attached to said riser string between said first buoy and said second end; and a second buoy seated on said landing ring is seated, said second buoy comprising at least one chamber for receipt of a buoyancy fluid.

A buoy system for an oil and gas marine riser string comprises: a first housing comprised of buoyant material, said first housing defined by a top wall, a bottom wall and a side wall joining said top and bottom walls, said walls defining an interior compartment within said housing, wherein said top wall is provided with an aperture and an opening is provided in at least a side wall or the bottom wall; and a second housing comprised of buoyant material, said second housing sized to pass through said opening in a wall of said first housing, said second housing having an aperture passing axially therethrough and at least one chamber for receipt of a buoyancy fluid.

An example of a buoyancy system for use with an offshore oil and gas riser string comprises: a buoyant collar comprising: an upper surface and a lower surface and a bore extending therebetween; a keyway disposed in said collar, substantially parallel to said bore, said keyway extending from said upper surface to said lower surface; and at least one chamber into which buoyancy fluid can be pumped.

An example of a method for installing an offshore oil and gas riser string between a rig at the surface of the ocean and a wellhead adjacent the ocean floor comprises: securing a first buoy having a second buoy removably disposed therein adjacent the ocean floor; attaching a blowout preventer to the lower end of a riser string; attaching a landing ring on said riser string above said blowout preventer; attaching a third buoy on said riser string above said landing ring; attaching a riser disconnect joint in said riser string above said third buoy; attaching riser joints above said riser disconnect joint; positioning said riser string adjacent said first buoy; moving said riser string so as to engage the second buoy; moving said riser string with the engaged second buoy away from said first buoy; and attaching said blowout preventer adjacent a wellhead on the ocean floor.

An example of a method for moving an offshore oil and gas rig attached to a riser extending down to the ocean floor and attached to a blowout preventer comprises: providing a riser with a first buoy secured adjacent a disconnect joint and a second buoy secured to said riser below said first buoy, wherein said riser system has an upper riser portion above said disconnect joint and a lower riser portion below said disconnect joint; pumping a fluid into said second buoy so as to increase the buoyancy of said second buoy and thereby the tension on the lower riser portion of said riser string; and disconnecting said rig and upper riser portion from said lower riser portion at said disconnect joint.

An example of a passive buoyancy system for attaching to and providing buoyancy for a marine riser comprises: a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser; a plurality of stackable elements stacked on the support mandrel; wherein each stackable element comprises a buoyant material; and wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto the support mandrel to affix to the support mandrel.

An example of a method for assembling a passive buoyancy system for integration of the passive buoyancy system into a marine riser comprises: providing a disc handling device wherein the disc handling device comprises a frame, and a plurality of lifting arms attached to the frame with a means for interfacing with, retrieving, lifting, and rotating a plurality of stackable elements from a support spool and a means for rotating and lowering the stackable elements onto a support mandrel; providing a transport spool; providing a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser; providing a plurality of stackable elements stacked on the transport spool wherein each stackable element comprises a buoyant material wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto a shaft wherein each stackable element comprises an orientation element for interfacing and locking with an adjacently stacked stackable element; interfacing each stackable element with the plurality of lifting arms; lifting each stackable element a distance apart from each adjacently stacked stackable element; rotating each stackable element so as to configure each keyway into alignment with one another; sliding the support mandrel through the aligned keyways of each stackable element so as to dispose the support mandrel substantially within the bore of each stackable element; rotating each stackable element to an angle that allows the orientation notch of each stackable disc to interface and lock with each adjacently stacked stackable element; and lowering and loading each stackable element onto the support mandrel.

An example of a method of assembling a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor comprises the steps of: providing a marine riser system comprising: a riser comprising an upper riser and a lower riser, a disconnect joint removably attaching the upper riser to the lower riser, and a hang-off ring attached to the lower riser; providing a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is moored to the seabed by one or more mooring lines wherein the second buoy is configured to mate with the hang-off ring of the lower riser; mating the marine riser system to the second buoy via the hang-off ring; introducing a buoyancy fluid into the at least one chamber of the second buoy; and disconnecting the upper riser from the lower riser at the disconnect joint.

The features and advantages of the present invention will be apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying figures, wherein:

FIG. 1A is a schematic depiction of an example of a conventional marine riser connecting an offshore rig to a wellhead.

FIG. 1B is a schematic depiction of a conventional marine riser having a plurality of buoyancy foam modules and an aircan integrally installed in the marine riser.

FIG. 2 is a schematic overview of a buoyancy tensioning system for a marine riser in accordance with one embodiment of the present invention.

FIG. 3A is a cross-sectional view of an active buoyancy element comprised of a buoyancy collar in accordance with one embodiment of the present invention.

FIG. 3B is a cross-sectional view of the active buoyancy element of FIG. 3A taken along the cut line 3B.

FIG. 3C is a front view of the active buoyancy element of FIG. 3A.

FIG. 4 is a cross-sectional view of an active buoyancy element seated on a landing ring in accordance with one embodiment of the present invention.

FIG. 5A is an overhead cross-sectional view of an active buoyancy element having an open keyway in accordance with one embodiment of the present invention.

FIG. 5B is an overhead cross-sectional view of an active buoyancy element having a restricted keyway in accordance with one embodiment of the present invention.

FIG. 6 is a cross-sectional view of an active buoyancy element having a vertically restricted keyway in accordance with one embodiment of the present invention.

FIG. 7A shows an overhead cross-sectional view of a garage buoy in accordance with one embodiment of the present invention.

FIG. 7B a cross-sectional view of a garage buoy of FIG. 7A taken along the cut line 7B in accordance with one embodiment of the present invention.

FIG. 8A is a cross-sectional view of a garage buoy having an active buoyancy element disposed therein in accordance with one embodiment of the present invention.

FIG. 8B is an overhead cross-sectional view of a garage buoy showing receipt of an active buoyancy element in accordance with one embodiment of the present invention.

FIG. 9A is a cross-sectional view of a garage buoy having an active buoyancy element disposed therein where the active buoyancy element features anti-rotation blocks to prevent the active buoyancy element from rotating with the garage buoy in accordance with one embodiment of the present invention.

FIG. 9B is an overhead cross-sectional view of a garage buoy showing receipt of an active buoyancy element having anti-rotation blocks in accordance with one embodiment of the present invention.

FIG. 9C is a cross-sectional view of the garage buoy of FIG. 9A but with the keyway hidden for riser storage in accordance with one embodiment of the present invention.

FIGS. 9D-9K illustrate an alternate embodiment of a buoyancy tensioning system featuring an SRTB moored to the seabed and a method of use thereof.

FIG. 10 is a side view of a passive buoyancy joint in accordance with one embodiment of the present invention.

FIG. 11 illustrates cross-sectional views of a buoyancy disc in accordance with one embodiment of the present invention.

FIG. 12A shows overhead cross-sectional views of alternating buoyancy discs so as to illustrate their principle of operation in accordance with one embodiment of the present invention.

FIG. 12B illustrates a side-view of a passive buoyancy mandrel joint in accordance with one embodiment of the present invention.

FIG. 13 further illustrates the principle of operation of stackable buoyancy discs in accordance with one embodiment of the present invention.

FIG. 14 illustrates a bottom support for installation of stackable buoyancy discs in accordance with one embodiment of the present invention.

FIG. 15 illustrates buoyancy discs lifted and separated on a transport spool in accordance with one embodiment of the present invention.

FIG. 16 illustrates stacked buoyancy discs with the transport spool removed in accordance with one embodiment of the present invention.

FIG. 17 illustrates stacked buoyancy discs rotated into a closed position in accordance with one embodiment of the present invention.

FIG. 18 illustrates the stacked buoyancy discs of FIG. 17 with a top support installed in accordance with one embodiment of the present invention.

FIGS. 19A and 19B are side and top views of a disc handling device for simultaneously manipulating a plurality of stackable buoyancy discs in accordance with one embodiment of the present invention.

FIGS. 20A and 20B are side and top views of a disc handling device shown in its initial configuration in accordance with one embodiment of the present invention (Sequence 1).

FIGS. 21A and 21B are side and top views of a disc handling device ready to interface with a plurality of buoyancy discs on a disc transport spool where the buoyancy discs are in a nested configuration in accordance with one embodiment of the present invention (Sequence 2).

FIGS. 22A and 22B are side and top views of a disc handling device with each disc handling stage preparing to grip each buoyancy disc in accordance with one embodiment of the present invention (Sequence 3).

FIGS. 23A and 23B are side and top views of a disc handling device with each disc handling stage engaging the buoyancy discs with lifting fingers rotated into lifting pockets of the buoyancy discs in accordance with one embodiment of the present invention (Sequence 4).

FIGS. 24A and 24B are side and top views of a disc handling device raising and separating buoyancy discs by extending lifting pistons on each disc lifting plate in accordance with one embodiment of the present invention (Sequence 5).

FIGS. 25A and 25B are side and top views of a disc handling device raising each buoyancy disc out of its nested configuration and rotating alternating discs in alternating directions in accordance with one embodiment of the present invention (Sequence 6).

FIGS. 26A and 26B are side and top views of a disc handling device with the transport spool removed in accordance with one embodiment of the present invention (Sequence 7).

FIGS. 27A and 27B are side and top views of a disc handling device with a mandrel joint aligned axially with the center of the buoyancy discs in accordance with one embodiment of the present invention (Sequence 8).

FIGS. 28A and 28B are side and top views of a disc handling device with alternating buoyancy discs rotated in opposite directions so as to align the interlocking segments with the open slots of the adjacent discs in accordance with one embodiment of the present invention (Sequence 9).

FIGS. 29A and 29B are side and top views of a disc handling device with each of the buoyancy discs lowered so as interlock in a nested configuration by collapsing the lifting pistons in accordance with one embodiment of the present invention (Sequence 10).

FIGS. 30A and 30B are side and top views of an alternate design of a disc handling device that utilizes hydraulic pins instead of rotating fingers to engage the buoyancy discs in accordance with one embodiment of the present invention.

FIGS. 31A and 31B are side and top views of a disc handling device with an alternate design that utilizes slotted lifting rods to raise and lower the buoyancy discs in accordance with one embodiment of the present invention.

FIG. 32A further illustrates the principle of operation of interlocking buoyancy discs with raised segments.

FIG. 32B is a circumferential side profile view of an alternate buoyancy disc design employing constantly sloping surfaces.

FIGS. 33A, 33B, 33C, 33D, 33E, and 33F illustrate a schematic overview depiction of a buoyancy tensioning system for a marine riser and installation thereof in accordance with one embodiment of the present invention.

FIGS. 34A, 34B, and 34C illustrate one example of a method for removal of an upper riser while leaving a lower riser in place in accordance with one embodiment of the present invention.

The Figures depicted herein are schematic depictions, and it is recognized that not all of the components depicted therein are drawn to scale.

While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention generally relates to buoyancy tensioning systems for offshore marine risers and more particularly, buoyancy tensioning systems for marine offshore riser systems that incorporate passive and active buoyancy elements and corresponding methods of use.

Generally, certain embodiments of the present invention provide devices and methods for the enhanced assembly and disassembly of offshore marine risers through the use of a combination of passive and active buoyancy elements in a marine riser. Individual elements of this overall system form additional embodiments of the present invention. Methods of use and corresponding methods of operation are also provided herein.

Advantages of the devices of the present invention include, but are not limited to, enhanced assembly and disassembly of offshore risers, particularly in the event of an emergency evacuation. Additionally, in certain embodiments, devices and methods of the present invention allow offshore rigs to be evacuated from an offshore riser without having to disassemble the entire riser or otherwise abandon the riser in place, which allows for a quicker disconnect and reconnect times. Furthermore, methods of certain embodiments of the present invention avoid the necessity of active buoyancy modules to be handled and stored on an offshore rig.

To facilitate a better understanding of the present invention, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the invention.

I. Overview of Elements of Buoyancy Tensioning System

FIG. 2 is a schematic overview of a buoyancy tensioning system for a marine riser in accordance with one embodiment of the present invention.

Riser 271 is a marine riser that extends from rig floor 251 to wellhead 277. Riser 271 allows hydrocarbons and drilling fluids to flow between wellhead 277 and an to an offshore rig at the ocean surface. Tensioning system 252 provides an upward tension on riser 271 so as to maintain riser 271 in a substantially vertical orientation and to prevent excessive bending loads on the wellhead 277.

Riser 271 is comprised of upper riser section 271A and lower riser section 271B. Near surface disconnect package (NSDP) 210 allows upper riser section 271A to be disconnected from lower riser section 271B. NSDP 210 is comprised of upper NSDP 210A and lower NSDP 210B. In the event of a disconnect, upper NSDP 210A remains with upper riser section 271A, whereas lower NSDP 210B remains with lower riser section 271B. Thus, in the event of an evacuation of an offshore rig and its corresponding upper riser section, for example, upper riser section 271A may be moved to another location independently of lower riser section 271B, which may be left in place. As lower riser section 271B is at a sufficient depth where it is not exposed to significant lateral water currents, it may safely be abandoned in place until upper riser section 271A can return and join again with lower riser section 271B.

For lower riser section 271B to be abandoned in place, however, lower riser section 271B requires additional buoyancy to prevent it from essentially “falling over,” which would constitute a catastrophic failure, likely resulting in significant loss of or damage to equipment and possible environmental exposure of hydrocarbons to the environment.

Lower riser section 271B is supported vertically in part by both passive buoyancy joint 220 and submerged riser tensioning buoy (SRTB) 230. SRTB 230 comprises an active buoyancy element, which comprises a buoyancy chamber which may be charged with a gas or other buoyant fluid. In normal use, while upper riser 271A and lower riser 271B are connected, SRTB 230 may be left deactivated (e.g. by flooding its chambers with seawater). In such a configuration, buoyancy is provided by tensioning system 252, passive buoyancy joint 220, and any other passive buoyancy elements incorporated into or on riser 271.

In the event of a planned disconnect of upper riser 271A and lower riser 271B, passive buoyancy joint 220 may be sized to maintain lower riser 271B in a substantially vertical position while additional buoyancy may be provided by SRTB 230 so as to reduce bending loads on wellhead 277. In this way, the combination of passive buoyancy joint 220 and SRTB 230 provides buoyancy to maintain lower riser 271B in a substantially vertical position regardless of whether lower riser 271B is connected to tensioning system 252 by way of upper riser 271A. In certain embodiments, SRTB may also comprise one or more passive buoyancy elements. In certain other embodiments, the SRTB may be used without passive buoyancy joint 220 if use of the passive buoyancy joint is deemed unnecessary.

In certain embodiments, buoyancy elements incorporated into riser 271 are sized such that riser 271 is slightly negatively buoyant or neutrally buoyant. Such a configuration is usually desirable to avoid the undesirable consequences that would result from a failure of a riser joint or connection with a positively buoyant riser upon 252. That is, if riser 271 were significantly positively buoyant, riser 271 could “rocket” or “shoot” upwards through the offshore rig upon failure, i.e., separation of a riser joint or riser connection which at the time of the failure is being held under high tension by tensioning system 252.

Lower Marine Riser Package 275 (LMRP) provides a surface-operable connection to blow out preventer (BOP) stack 276. Blow out preventer (BOP) stack 276 in turn interfaces with wellhead 277 and provides fluid isolation between riser 271 and wellhead 277 as desired.

Garage buoy 260 provides a housing for SRTB 230 for storage and constrains SRTB 230 when SRTB 230 is not in use. Garage buoy, sometimes referred to herein as the “third buoy,” houses SRTB 230 before SRTB 230 is installed on lower riser 271B or any other time when storage of SRTB 230 is desired. Additionally, garage buoy 260 houses SRTB 230 after removal of SRTB 230 from lower riser 271B. Garage buoy may be fixed in proximity to wellhead 277 by mooring lines 279 attached to anchors 278.

Each component of marine buoyancy marine tensioning system 200 is explained in more detail below. Methods of corresponding use are also provided below in more detail below.

II. Submerged Riser Tensioning Buoy (SRTB) and Garage Buoy Concept

FIG. 3A is a cross-sectional view of an active buoyancy element comprised of a buoyancy collar in accordance with one embodiment of the present invention. FIG. 3B is a cross-sectional view of the active buoyancy element of FIG. 3A taken along cut line 3B. FIG. 3C is a front view of the active buoyancy element of FIG. 3A. Active buoyancy elements are also referred to herein as submerged riser tensioning buoys (SRTB) or simply, the “second buoy.”

Submerged riser tensioning buoy (SRTB) 230 comprises an active buoyancy component, which provides additional buoyancy to a riser when connected thereto. In this example, the active buoyancy components are air chambers 320, which may be “activated” by charging with a gas or other buoyant fluid to provide additionally buoyancy to SRTB 230. Alternatively, air chambers 320 may be flooded with seawater to “deactivate” SRTB 230 so as to neutralize the positive buoyancy of SRTB 230. Charging of air chambers 320 may be accomplished through valves 310. Ports 331 allow communication among air chambers 320 or to the environment (e.g. nitrogen or air may be released through an egress port). Ports 331 may also allow each air chamber to be isolated from one another to allow independent charging or operation of each air chamber.

Generally, SRTB 230 provides additionally buoyancy during various uses of riser 271. For example, SRTB 230 may be activated in the event of a disconnect of upper riser 271A and lower riser 271B. SRTB 230 may be activated at any time when additional buoyancy to lower riser 271B is desired. In certain embodiments, SRTB 230 may also incorporate passive buoyancy components into its structure. For example, SRTB 230 may include passive buoyancy components in walls 350.

SRTB 230 mates or otherwise interfaces with lower riser 271B by landing profile 340. Landing profile 340 is a seat that is adapted to mate with a hang-off ring (see e.g., 480 of FIG. 4) of lower riser 271B. In this way, landing profile 340 assists with the docking and interfacing of SRTB 230 with lower riser 271B by providing a seat or profile that preferentially interfaces with a corresponding structure installed on lower riser 271B.

SRTB 230 features keyway 360, which provides a slot for unidirectionally interfacing with lower riser 271B. FIG. 4 shows the interaction of a keyway with a hang-off ring in more detail for an alternate embodiment of an active buoyancy element.

FIG. 4 is a cross-sectional view of an active buoyancy element seated on a landing ring in accordance with one embodiment of the present invention. Here, hang-off ring 480 is shown attached to lower riser 471B. Hang-off ring 480 seats on or otherwise interfaces with landing profile 440. Centralizer ring 470 preferentially positions lower riser 471B horizontally in keyway 460. In this way, the structure of SRTB 400 preferentially seats or otherwise mates with corresponding elements affixed to lower riser 471B.

FIG. 5A is an overhead cross-sectional view of an active buoyancy element having an open keyway in accordance with one embodiment of the present invention. The open, non-restricted keyway 563 of SRTB 501 allows unrestricted lateral engagement of SRTB 501 with a lower riser. FIG. 5B is an overhead cross-sectional view of an active buoyancy element having a restricted keyway in accordance with one embodiment of the present invention. Unlike the open, non-restricted keyway 563 of FIG. 5A, restricted keyway 567 constrains movement of SRTB 502 once engaged to a lower riser. In this way, the physical constriction provided by restricted keyway 567 prevents inadvertent release of SRTB 502 from a riser reducing the risk of decoupling from a riser due to water current forces.

FIG. 6 is a cross-sectional view of an active buoyancy element having a vertically restricted keyway in accordance with one embodiment of the present invention. Here, centralizer mating profile 674 provides a corresponding seating surface for interfacing with centralizer ring 670. Once engaged, the combination of centralizer ring 670 and centralizer mating profile 674 inhibits undesired vertical movement.

FIG. 7A shows an overhead cross-sectional view of a garage buoy in accordance with one embodiment of the present invention. Garage buoy 703, also referred to herein as the “third buoy,” is adapted to house an SRTB when the SRTB is not in use. That is, when the SRTB is not attached to a riser, garage buoy 703 houses and constrains movement of the SRTB. Garage buoy 703 has a U-shaped cut-out or slot (garage buoy slot 765) adapted to allow passage of a riser therethrough.

FIG. 7B is a cross-sectional view of a garage buoy of FIG. 7A taken along the cut line 7B in accordance with one embodiment of the present invention. This view shows one face of garage buoy 703 open for receipt of an SRTB. Although depicted here with a rectangular configuration, it is explicitly recognized herein that garage buoy 703 may take any shape suitable to house an SRTB. In certain preferred embodiments, suitable garage buoys will additionally allow partial passage of a riser through the garage buoy to allow interfacing of a riser to an SRTB housed in the garage buoy.

In certain embodiments, garage buoy 703 has a positive buoyancy and is moored to the ocean floor or other fixed structure in proximity to the ocean or sea floor. Garage buoy 703 may be preinstalled well before the arrival or assembly of a corresponding marine riser. This preinstallation of the SRTB allows the SRTB to be delivered and installed by a vessel separate from the offshore rig containing the marine riser elements. As mentioned previously, this preinstallation of garage buoy 703 and its corresponding SRTB is advantageous in that the garage buoy and the SRTB need not be stored on the offshore rig, which in effect eliminates a significant volume of equipment from the offshore rig.

In certain embodiments, SRTBs of the present invention are one or more gas chambers integrally formed as one unit or buoyancy module with an overall diameter or overall size of about 24 feet to about 48 feet, and in certain embodiments, about 28 feet. Because SRTBs of the present invention are deployed separately from the marine riser, i.e. not from the rig itself, SRTBs of the present invention are logistically much simpler and safer due to this reduction in equipment handling on the rig.

FIG. 8A is a cross-sectional view of a garage buoy with an SRTB having an active buoyancy element disposed therein in accordance with one embodiment of the present invention. Garage buoy 803 houses SRTB 801. FIG. 8B is an overhead cross-sectional view of a garage buoy showing receipt of active buoyancy element SRTB 801. SRTB 801 is shown being introduced into garage buoy 803. Here, keyway 863 is shown aligned with garage buoy slot 865. This aligned position allows SRTB 801 to accept a riser for engagement with SRTB 801 (as illustrated and explained in more detail below).

FIG. 9A is a cross-sectional view of a garage buoy having an active buoyancy element disposed therein, where the active buoyancy element features anti-rotation blocks to prevent the active buoyancy element from rotating within the garage buoy in accordance with one embodiment of the present invention. In this way, anti-rotation blocks 905 maintains alignment of keyway 963 and slot 965.

Anti-rotation blocks 905 may take any shape suitable for constraining the rotational movement of an SRTB disposed in garage buoy 905. FIG. 9B is an aerial cross-sectional view of a garage buoy showing receipt of an active buoyancy element, having anti-rotation blocks in accordance with one embodiment of the present invention. Here, anti-rotation blocks 905 are shown as affixed to SRTB 901 rather than being affixed to or integral to garage buoy 903. Similar to the alignment depicted in FIG. 8, keyway 963 of SRTB 901 is aligned with slot 965, which allows for receipt of a riser for engagement (as illustrated and explained in more detail below).

FIG. 9C is a cross-sectional view of the garage buoy of FIG. 9A but with the keyway hidden for riser storage in accordance with one embodiment of the present invention. In this case, SRTB 901 is rotated so that keyway 963 and slot 965 are not aligned. Hidden keyway 963 is advantageous in that, given sufficient positive buoyancy in the SRTB 901 and garage buoy 903, the marine riser may be “hung off”, or suspended from the SRTB 901 with the keyway effectively closed off, thus reducing the risk of the marine riser accidentally disengaging from the SRTB; e.g., in the case of a partially-flooded and inclined SRTB 901 and/or with high currents acting on the riser that was left suspended.

FIGS. 9D-9K illustrate an alternate embodiment of a buoyancy tensioning system featuring an SRTB moored to the seabed and a method for use thereof. Instead of the submerged riser tensioning buoy being free-floating, SRTB 901 may be moored to anchors 978. This configuration contrasts with the previous embodiments, which showed SRTB 901 normally attached to the lower riser or “parked” in the garage buoy.

For certain deepwater wells, the submerged riser tensioning buoy may be moored directly over or in close proximity to the well location as an alternative to deploying the SRTB from a moored garage buoy. In this embodiment, mooring lines 979 could remain attached to SRTB 901 throughout the period of operations, especially for exploration or appraisal wells where operating at the well location is not complicated by the installation or existence of subsea facilities, and where there is not a need for intense, multiple ROV support.

FIGS. 9D and 9E illustrate plan and elevation views of an SRTB moored to the sea floor. Here, SRTB 901 is ready to receive a riser (not shown) for connection to wellhead 977.

FIGS. 9F and 9G illustrate an SRTB ready to receive and interface with a riser. Here, riser 971 is being moved into position to interface with SRTB 901. Spring buoys 935 add additional tension on mooring lines 938 to maintain mooring lines 979 in tension, particularly when riser 971A is lowered to its “connected” position as described further below. In one embodiment, SRTB 901 is anchored directly to the seabed using multiple mooring lines 935 instead of being deployed from the garage buoy. SRTB 901 is anchored such that it is positioned directly over or in close proximity to the intended well head 977.

FIGS. 9H and 9I illustrate SRTB 901 engaged with riser 971 and being moved into position for connection to wellhead 977.

FIGS. 9J and 9K illustrate riser 971 connected to wellhead 077. Each mooring line 979 in this embodiment includes one or more spring buoys 935 positioned along its length such that when the lower riser 971B is connected to the subsea wellhead 977, any slack created in mooring line 979 as a result of the further submergence or deactivation of SRTB 901 from its original position in the water column is taken up by these spring buoys 935, so as to maintain sufficient tautness in the upper portions of mooring lines 979 to prevent fouling of mooring lines 935 around the riser or its attachments. In certain embodiments, mooring lines 979 may be poly moorings.

So as not to affect the proper functioning of an emergency disconnect system (EDS), a weak-point 936 or disconnect 936 could be introduced into each mooring line 979 to ensure that if moorings lines 979 failed, before any damage is done to riser 971 and/or SRTB 901 in the event of a drive-off/drift-off. Alternately, mooring lines 979 may be disconnected upon mating of riser 971/SRTB 901 and then later reconnected when LMRP 975 is pulled.

III. Passive Buoyancy Joint and Disc Handling Device

FIG. 10 is a side view of a passive buoyancy joint in accordance with one embodiment of the present invention. Passive buoyancy joint 1020, also referred to herein as the “first buoy,” illustrates passive buoyancy joint 220 of FIG. 2 in more detail. Passive buoyancy joint 1020 is formed from a plurality of stacked buoyancy discs 1021. Buoyancy discs 1021 are bounded by top support 1023 and bottoms support 1027, which provide support for and constrain buoyancy discs 1021 on riser 1071.

Passive buoyancy joint 1020 aids in maintaining a lower riser in the vertical position. In certain embodiments, passive buoyancy joint 1020 is placed in proximity to the top of the lower riser. The passive buoyancy joint 1020 is optional and may be excluded from certain embodiments of the present invention.

The modular design utilizing stackable buoyancy discs facilitates easier handling and storage of the discs at the rig and allows for easier reconfiguration of the total buoyancy of passive buoyancy joint 1020. Additionally, each buoyancy disc 1021 is formed so as to allow an interlocking fit with an adjacent buoyancy disc.

Although passive buoyancy joint 1020 is depicted herein as formed of a plurality of buoyancy discs 1021, it is explicitly recognized that passive buoyancy joint 1020 may be formed of one integral component. Passive buoyancy joint 1020 may be constructed out of any material suitable for providing a positive buoyancy, including, but not limited to, foam, closed-cell foam, any lightweight material having a density less than that of seawater, or any combination thereof.

FIG. 11 illustrates cross-sectional views of a buoyancy disc 1021 in accordance with one embodiment of the present invention. Keyway 1063 allows buoyancy disc 1021 to be laterally mounted on a riser. Raised sections 1068 allow each buoyancy disc 1021 to interlock with other adjacent buoyancy discs. In certain embodiments, buoyancy disc 1021 may have an outside diameter from about 5 feet to about 15 feet. In certain embodiments, dimension D2 is preferably about twice the dimension D1.

FIG. 12A shows overhead cross-sectional views of alternating buoyancy discs so as to illustrate their principle of operation in accordance with one embodiment of the present invention. Alternating stackable buoyancy discs 1021A and 1021B are shown disposed about mandrel joint 1071. Mandrel joint 1071 includes an orienting protrusion 1072, which facilitates positioning of alternating buoyancy discs 1021A and 1021B. Buoyancy disc 1021A has been rotated counterclockwise until abutting up against orienting protrusion 1072 (refer to reference axis 1069 of 1021A for rotational reference). Buoyancy disc 1021B has been rotated clockwise until abutting up against orienting protrusion 1072 (refer to reference axis 1069 of 1021B for rotational reference).

In this way, each adjacent buoyancy disc may be rotated in an alternating fashion so as to allow each adjacent disc to be positioned to facilitate each buoyancy disc nesting and interlocking with one another. That is, raised sections 1068 of buoyancy disc 1021A interface with keyway 1063 of buoyancy disc 1021B when each alternating buoyancy disc is stacked on top of an adjacent disc. Likewise, raised sections 1068 of buoyancy disc 1021B interface with keyway 1063 of buoyancy disc 1021A.

In certain embodiments, a buoyancy wedge (not shown), constructed of buoyant material, may be added to fill in the void space of keyway 1063.

FIG. 12B illustrates a side-view of a passive buoyancy mandrel joint in accordance with one embodiment of the present invention. Mandrel joint 1071 includes bottom support notch 1024 and top support notch 1028. Bottom support notch 1024 and top support notch 1028 may be used to support top and bottom supports such as, for example, top support 1023 and bottoms support 1027 of FIG. 10.

Orienting protrusion 1072, as previously explained, provides a rotational stop or limit to assist in orienting each disc to its final desired rotational position.

FIG. 13 further illustrates the principle of operation of stackable buoyancy discs in accordance with one embodiment of the present invention. Here, buoyancy disc 1021 is shown at its maximum rotational limit by virtue of keyway 1063 abutting up against orienting protrusion 1072. Depending on whether buoyancy disc is rotated clockwise or counterclockwise, buoyancy disc 1021 will abut against orienting protrusion 1072 so as to situate in either a clockwise configuration indicated by reference axis 1069A or a counterclockwise configuration indicated by reference axis 1069B. In this way, buoyancy discs may be mounted on a mandrel with each buoyancy disc oriented in an alternating fashion so as to allow each buoyancy disc to interlock with an adjacent buoyancy discs.

FIG. 14 illustrates a bottom support for installation of stackable buoyancy discs in accordance with one embodiment of the present invention. Bottom support 1027 is affixed to mandrel joint 1071 and is ready to accept buoyancy discs 1027 stacked on mandrel joint 1071. Typically, bottom support 1027 will be added to mandrel joint in the rig moon pool.

With reference to FIG. 15, before installation of buoyancy discs 1021 on a mandrel joint for integration with a marine riser, buoyancy discs 1021 may be stored on transport spools 1073 when not in use. A transport spool may be used to store buoyancy discs before use. FIG. 15 illustrates buoyancy discs lifted and separated on a transport spool in accordance with one embodiment of the present invention.

As each buoyancy disc 1021 has been rotated in an alternating fashion, the series of discs 1021 cannot all be removed simultaneously from transport spool 1073 by simultaneously displacing all buoyancy discs 1021 in one direction. This configuration, where each alternating buoyancy disc is rotated in an alternating fashion such that each keyway is not aligned with the keyways of adjacent discs, may be referred to herein as the “closed configuration” or “nested configuration.” The simultaneous removal of buoyancy discs 1063 is prevented because keyways 1063 of each disc 1021 are not aligned with one another as shown in FIG. 15. Accordingly, before removal of buoyancy discs 1021, each disc must be lifted, separated from one another, and then rotated in an alternating fashion so as to align each keyway 1063 with one another so as to facilitate removal of each buoyancy disc 1021 from transport spool 1073 (see e.g., FIG. 16). This permits buoyancy discs 1021 to be stored on transport spool 1073 and for transport spool 1073 to be readily handled and moved without risk having discs fall off of transport spool 1073.

FIG. 16 illustrates stacked buoyancy discs rotated and aligned so as to facilitate removal of the transport spool in accordance with one embodiment of the present invention. As shown in this figure, buoyancy discs 1021 have been rotated, each in an alternating fashion such that the keyways of each buoyancy disc 1021 are aligned so as to allow removal of a transport spool (not shown). This configuration, where each keyway is aligned with the keyways of adjacent discs, may be referred to herein as the “open configuration.”

After removal from a transport spool, buoyancy discs may be installed on a mandrel joint with all of the buoyancy discs in the open configuration. After mounting each buoyancy disc onto the mandrel joint, each disc may then be rotated in an alternating fashion so as to place the buoyancy discs in a closed configuration. As an example of such a closed configuration, FIG. 17 illustrates stacked buoyancy discs rotated into a closed position in accordance with one embodiment of the present invention. Here, each buoyancy disc 1021 has been rotated in an alternating fashion so as to place the buoyancy discs 1021 in a closed configuration. Each buoyancy disc 1021, however, is still lifted and separated from each adjacent disc. That is, in this figure, each buoyancy disc 1021 has not yet nested with each adjacent disc by allowing each disc to drop into and nest with each adjacent disc.

FIG. 18 illustrates the stacked buoyancy discs of FIG. 17 with a top support installed in accordance with one embodiment of the present invention. Here, each buoyancy disc 1021 has been allowed to drop and nest with each adjacent disc. In this way, each disc is interlocked with each adjacent disc in the closed configuration. Top support 1023 and bottom support 1027 support and constrain buoyancy discs 1021 in place on riser mandrel joint 1071.

IV. Buoyancy Disc Handling Devices

Although each buoyancy disc may be individually handled and manipulated, in certain embodiments, it is advantageous and more efficient to manipulate multiple buoyancy discs simultaneously. Examples of operations suitable for multiple simultaneous disc manipulation include, but are not limited to transferring buoyancy discs from a transport spool to a riser and vice-versa.

An example of a device suitable for simultaneously manipulating a plurality of buoyancy discs is shown in FIGS. 19A and 19B, referred to herein as a disc handling device. More particularly, FIGS. 19A and 19B are side and top views of a disc handling device for manipulating a plurality of stackable buoyancy discs off a disc transport spool in accordance with one embodiment of the present invention. FIGS. 20-30 describe in detail an example of a sequence for simultaneously installing multiple buoyancy discs from a disc transport spool to a mandrel joint.

Turning back to FIGS. 19A and 19B for a brief description of the principle components of disc handling device 2000, disc handling device 2000 comprises a plurality of lifting plates 2083. Each lifting plate 2083 is configured to interface with and manipulate a buoyancy disc 2021. In this example, lifting plate 2083 comprises arms 2084 that extend outwardly around a buoyancy disc. The arms 2084 are guided by guide track 2085 for circumscribing a portion of the circumference of a buoyancy disc. Arms 2084 have lifting fingers 2086 for interfacing or otherwise latching into buoyancy disc lifting pockets 2087. In this way, arms 2084 and lifting fingers 2086 can “grab” or engage each buoyancy disc 2021 and individually manipulate (e.g. rotate, raise, and lower) each buoyancy disc 2021.

Lifting plates 2083 are raised and lowered by lifting pistons 2082. Lifting pistons 2082 may be hydraulically or mechanically actuated. In this way, lifting plates 2083 are capable of lifting adjacent, nested buoyancy discs 2021 so as to separate them.

Normally, when each disc rests on a transport spool, each buoyancy disc is nested and interlocked with adjacent discs (refer to FIG. 12A and the corresponding discussion for a description of interlocking of raised sections with keyways of adjacent discs). Additionally, in this nested configuration, because each buoyancy disc is rotated out of alignment with respect to each adjacent disc, the keyways of each disc are not aligned. Therefore, the nested discs are prevented from being simultaneously being removed from the transport spool because the keyways are out of alignment with one another. Before the buoyancy discs may be removed from a transport spool, each disc must be rotated in an alternating fashion so as to align their keyways so as to allow removal of the buoyancy discs from the transport spool.

The buoyancy discs, however, are prevented from rotating in their nested configuration because of the interlocking of the raised sections of each buoyancy disc with the keyways of adjacent buoyancy discs. Accordingly, before the buoyancy discs can be rotated, each disc must be raised and separated from adjacent buoyancy discs. Accordingly, to effect the simultaneous removal of a plurality of buoyancy discs from a transport spool for installation on a mandrel joint or riser, the buoyancy discs on the transport spool must be raised and separated from one another. Each disc must then be rotated in an alternating fashion so as to align their keyways. This permits the buoyancy discs to be removed from the transport spool and mounted on the mandrel joint. Each disc may then be rotated in an alternating fashion until the raised section of each buoyancy disc aligns with the keyways of adjacent discs. Finally, each disc is lowered onto one another so that each buoyancy disc interlocks with adjacent discs in a nested configuration. FIGS. 20-30 describe in detail an example of this sequence for simultaneously manipulating a plurality of buoyancy discs from a disc transport spool to a mandrel joint.

FIGS. 20A and 20B are side and top views of a disc handling device shown in its initial configuration in accordance with one embodiment of the present invention (Sequence 1). In these figures, disc handling device 2000 is shown in its initial configuration, ready to interface with a plurality of buoyancy discs. In this configuration, disc handling device 2000 may be skidded into position for gripping a plurality of buoyancy discs in the nested configuration. Alternatively, disc handling device 2000 may rest stationary while a plurality of buoyancy discs are moved into position for interfacing with disc handling device 2000.

FIGS. 21A and 21B are side and top views of a disc handling device ready to interface with a plurality of buoyancy discs carried on a disc transport spool, where the buoyancy discs are in a nested configuration in accordance with one embodiment of the present invention (Sequence 2). Here, buoyancy discs 2021 are resting in their nested configuration, where each buoyancy disc is interlocked with adjacent buoyancy discs. Buoyancy discs 2021 have been moved into position for interfacing with disc handling device 2000. Each lifting plate 2083 is vertically aligned with each buoyancy disc 2021.

FIGS. 22A and 22B are side and top views of a disc handling device with each disc handling stage preparing to grip each buoyancy disc in accordance with one embodiment of the present invention (Sequence 3). Here, arms 2084, guided by guide track 2085, are extended by arm extension pistons 2088 so as to circumscribe a portion of the circumference of buoyancy discs 2021. Arms 2084 extend a distance about buoyancy discs 2021 sufficient to position lifting fingers 2086 in proximity to lifting pockets 2087.

FIGS. 23A and 23B are side and top views of a disc handling device with each disc handling stage engaging the buoyancy discs with lifting fingers rotated into lifting pockets of the buoyancy discs in accordance with one embodiment of the present invention (Sequence 4). Here, arms 2084 have extended sufficiently to locate lifting fingers 2086 into position adjacent to lifting pockets 2087 of buoyancy discs 2021. Lifting fingers 2086 engage lifting pockets 2087 of buoyancy disc 2021 for manipulation of buoyancy disc 2021. In this way, disc handling stage 2089 can raise, lower, and rotate buoyancy disc 2021.

In FIGS. 23A and 23B, buoyancy discs 2021 are still in their nested configuration, i.e., none of the buoyancy discs 2021 have been raised.

FIGS. 24A and 24B are side and top views of a disc handling device raising and separating buoyancy discs by extending lifting pistons on each disc lifting plate in accordance with one embodiment of the present invention (Sequence 5). Here, lifting pistons 2082 have raised each buoyancy disc 2021 partially out of their nested configuration.

FIGS. 25A and 25B are side and top views of a disc handling device raising each buoyancy disc out of its nested configuration and rotating alternating discs in alternating directions in accordance with one embodiment of the present invention (Sequence 6). Here, lifting pistons 2082 have raised each buoyancy disc 2021 fully out of their nested configuration. In this way, buoyancy discs 2021 are no longer interlocked with one another so that they are free to rotate without restriction. Before rotation of buoyancy discs 2021, however, keyways 2063 of each disc are out of alignment with the keyways of adjacent discs. In this figure, the topmost buoyancy disc is shown as having been rotated about 45° in preparation for removal of transport spool 2073. Rotation of buoyancy discs 2021 is accomplished by actuation of arm extension pistons 2088 which extend and retract arms 2084.

FIGS. 26A and 26B are side and top views of a disc handling device with the keyways of each buoyancy disc 2021 aligned for removal of the transport spool in accordance with one embodiment of the present invention (Sequence 7). Here, each buoyancy disc 2021 has been rotated such that each keyway 2063 of each buoyancy disc 2021 has been rotated in alternating directions so that keyways 2063 are all in alignment with one another. This alignment of keyways 2073 allows removal of transport spool 2073 (transport spool not shown in FIGS. 26A and 26B). Transport spool 2073 may be removed by either displacing transport spool 2073 away from disc handling device 2000 or vice-versa.

Now, buoyancy discs 2021 are ready for simultaneous installation on a mandrel joint as keyways 2063 are commonly aligned.

FIGS. 27A and 27B are side and top views of a disc handling device with a mandrel joint aligned axially with the center of the buoyancy discs in accordance with one embodiment of the present invention (Sequence 8). Here, mandrel joint 2071 has been moved into position so as to allow buoyancy discs 2021 to mount thereon. Each buoyancy disc 2021 may be rotated in alternating directions by arms 2084 while buoyancy discs 2021 are still in a fully raised configuration.

FIGS. 28A and 28B are side and top views of a disc handling device with alternating buoyancy discs rotated in opposite directions so as to align the interlocking segments with the open slots of the adjacent discs in accordance with one embodiment of the present invention (Sequence 9). Here, each buoyancy disc 2021 has been rotated in an alternating fashion about 45° to allow buoyancy discs 2021 to return to their nested configuration. Each buoyancy disc 2021 is shown as partially lowered into the nesting configuration.

FIGS. 29A and 29B are side and top views of a disc handling device with each of the buoyancy discs fully lowered so as to interlock the disks in a nested configuration by collapsing the lifting pistons in accordance with one embodiment of the present invention (Sequence 10). Here, buoyancy discs 2021 have been positioned in a nested configuration on mandrel joint 2071. Upon placement of buoyancy discs 2021 in the nested configuration, lifting fingers 2086 may be withdrawn, and arms 2084 retracted. Disc handling device 2000 may now be skidded away from mandrel joint 2071 to retrieve additional buoyancy discs if desired.

Each of the steps described in FIGS. 19-29 may of course be reversed to remove buoyancy discs from a mandrel joint and return the buoyancy discs to a transport spool for storage. The foregoing permits multiple buoyancy discs to be handled simultaneously or individually, as desired.

FIGS. 30A and 30B are side and top views of an alternate design of a disc handling device that utilizes hydraulic pins instead of rotating fingers to engage the buoyancy discs in accordance with one embodiment of the present invention. Hydraulic pins 2091 engage lifting pockets 2087. In this way, any suitable gripping mechanism may be utilized to engage buoyancy discs 2021. Additionally, gear drive 2092 rotates arms 2084. Likewise, any suitable rotation mechanism may be used to rotate arms 2084 for manipulation of buoyancy discs 2021.

FIGS. 31A and 31B are side and top views of a disc handling device with an alternate design that utilizes slotted lifting rods to raise and lower the buoyancy discs in accordance with one embodiment of the present invention. Lifting rods 2093 and 2094 feature lifting appendages 2095, which interface with slots 2095 so as to allow buoyancy discs 2021 to be raised and lowered as before. First lifting rods 2093 and second lifting rods 2094 are configured to rotate clockwise and counterclockwise as desired. In this way, first lifting rods 2093 can rotate in a direction opposite to second lifting rods 2094. Additionally, first lifting rods 2093 interface with every other buoyancy disc, and second lifting rods 2094 interface with the remaining buoyancy disc. Consequently, similar to earlier embodiments of the disc handling devices 2000 and 2001, disc handling device 2002 is capable of raising and lowering buoyancy discs 2021 and subsequently rotating each adjacent disc in an alternating fashion.

FIG. 32A further illustrates the principle of operation of interlocking buoyancy discs with raised segments by showing the side profile of the discs along their circumferences. As before, buoyancy discs 2021 feature raised sections 2063. The open configuration shows buoyancy discs 2021 raised and separated from one another, whereas the locked or nested configuration shows buoyancy discs 2021 interlocking with one another via the raised sections interfacing with the keyways of adjacent discs. In this embodiment, it is necessary to raise and lower the buoyancy discs before rotating them.

FIG. 32B is a circumferential side profile view of an alternate buoyancy disc design employing constantly sloping surfaces. Alternative buoyancy disc 2021 features constantly sloping surfaces. In this embodiment, raising the buoyancy discs prior to rotation is not necessary. Because the discs are feature constantly-sloping surfaces, the discs are self-lifting. Therefore, switching from the nested to the open position and vice-versa may be accomplished by merely rotating each disc or every other disc in an alternating fashion.

V. Methods

FIGS. 33A, 33B, 33C, 33D, 33E, and 33F illustrate a schematic overview depiction of a buoyancy tensioning system for a marine riser and installation thereof in accordance with one embodiment of the present invention. More particularly, FIG. 33A shows buoyancy tensioning riser system 3200 run from an offshore rig. Buoyancy tensioning riser system 3200 comprises BOP stack 3276, lower marine riser package (LMRP) 3275, lower riser 3271B, hang-off ring 3280, passive buoyancy joint 3220, lower near surface disconnect package (NSDP) 3210B, and upper near surface disconnect package (NSDP) 3210A. Each element is assembled in sequence and run from rig floor 3251 to form buoyancy tensioning riser system 3200.

As is apparent in FIG. 33A, submerged riser tensioning buoy (SRTB) 3230 is stored in garage buoy 3260, which is in turn preferably moored adjacent to the ocean floor via mooring lines 3279 attached to anchors 3278. In this configuration, SRTB 3230 is ready to interface with buoyancy tensioning riser system 3200. Hang-off ring 3280 on riser 3271 is configured to seat on or otherwise mate with corresponding structure on SRTB 3230.

As explained previously, garage buoy 3260 and SRTB 3230 have been previously been located in position ready to interact with buoyancy tensioning riser system 3200. This preinstallation may be accomplished by the same offshore rig 3251 or may be accomplished by a separate vessel. Preinstalling SRTB 3230 reduces the critical path for assembling buoyancy tensioning riser system 3200 and further reduces the volume of equipment that must be handled on offshore rig 3251.

In FIG. 33B, buoyancy tensioning riser system 3200 is moved over in proximity to garage buoy 3260 so as to interface directly with SRTB 3230. In so doing, hang-off ring 3280 is moved into position to seat on or otherwise mate with SRTB 3230.

FIG. 33C shows hang-off ring 3280 fully engaging or otherwise mating with a landing profile on SRTB 3230. Now that buoyancy tensioning riser system 3200 is fully engaged with SRTB 3230, SRTB 3230 may be undocked from garage buoy 3260.

FIG. 33D shows SRTB 3230 in the process of being undocked from garage buoy 3260. Once undocked from garage buoy 3230, buoyancy tensioning riser system 3200 may be moved over to wellhead 3277 for connection thereto. FIG. 33E shows buoyancy tensioning riser system 3200 being moved into position for mating with wellhead 3277. As buoyancy tensioning riser system 3200 is not yet connected to wellhead 3277, tensioning system 3252 is not yet activated.

FIG. 33F shows buoyancy tensioning riser system 3200 connected to wellhead 3277. Now that buoyancy tensioning riser system 3200 is connected to wellhead 3277, tensioning system 3252 may be activated to assist buoyancy tensioning riser system 3200 remaining vertical. In certain embodiments, SRTB 3230 is left deactivated, that is, not charged with a gas or buoyant fluid during normal operation.

FIGS. 34A, 34B, and 34C illustrate one example of a method for removal of an upper riser while leaving a lower riser in place in accordance with one embodiment of the present invention.

More particularly, FIG. 34A shows buoyancy tensioning riser system 3200 preparing for disconnection of upper riser 3271A from lower riser 3274B. In the event of a planned evacuation (not to be confused with an “emergency disconnect”, a term generally used in the industry to refer to the pre-programmed, semi-automatic sequence of closing the blind shear rams of the BOP, unlocking the LMRP connector, and lifting the upper and lower riser and LMRP using riser tensioners 3252, such “emergency disconnect” usually performed for emergencies such as a loss of dynamic positioning ability due to external meteorological or oceanographic phenomena or due to a failure of the onboard dynamic positioning systems), SRTB 3230 can be activated to provide additional buoyancy. That is, air chambers 3220 of SRTB 3230 may be charged with a gas or buoyant fluid. Charging of air chambers 3220 may be accomplished by any suitable method including charging via an air line, such as air line 3223 from vessel 3222 or from ROV 3224. Of course, air line 3223 could run directly from rig 3251 either independently or as an auxiliary line on the upper riser 3271A.

Once SRTB 3230 is charged, either fully or partially, near surface disconnect package (NDSP) 3210 can be disconnected so as to release upper riser 3271A from lower riser 3271B. In this way, SRTB 3230 provides the additional buoyancy support required to support lower riser 3271B in the upright, vertical position, plus any additional extra tension deemed necessary to maintain bending moments on the subsea wellhead at an acceptable magnitude.

FIG. 34B shows upper riser 3271A disconnected from lower riser 3271B. SRTB 3230 is activated and providing additional buoyancy to secure lower riser 3271B in the vertical position. Tensioning system 3252 may be released or otherwise deactivated now that upper riser 3271A is no longer connected to lower riser 3271B. Now, upper riser 3271A is disconnected from the lower riser 3271B, the rig 3251 is free to be evacuated or moved from the site as desired. FIG. 34C shows lower riser 3271B safely left behind, as it is not adversely affected by near surface conditions because the lateral water currents are minimal at the lower depths where lower riser 3271B is situated, or sufficient additional reserve tension is provided by the SRTB 3230 to maintain riser verticality in the presence of anticipated water currents at the lower depths.

The above steps may be reversed to reconnect rig 3251 and upper riser 3271A to lower riser 3271B so as to put buoyancy tensioning riser system 3200 in operation. Additionally, if desired, SRTB 3230 may be detached from lower riser 3271B as desired by reversing the steps detailed above as well.

As used herein, the terms “join,” “affix to,” “connect to,” and “attach to” do not require elements to be directly connected to other elements and include elements indirectly connected to one another so as to communicate mechanical forces.

It is explicitly recognized that any of the features, elements, and steps of each embodiment described herein may be combined with any other embodiment described herein.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. 

1. A method of retrieving a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor, the method comprising the steps of: providing a marine riser system comprising: a riser comprising an upper riser and a lower riser, a disconnect joint removably attaching the upper riser to the lower riser, and a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is non-integral to the lower riser and removably attached to the lower riser; introducing a buoyancy fluid into the at least one chamber of the second buoy; and disconnecting the upper riser from the lower riser at the disconnect joint.
 2. The method of claim 1 wherein the marine riser system further comprises a first buoy attached to the lower riser.
 3. The method of claim 1 wherein a sufficient quantity of buoyancy fluid is introduced into the second buoy to maintain the lower riser in a substantially vertical position to prevent the lower riser from collapsing.
 4. The method of claim 1 further comprising: retrieving the upper riser; and leaving the lower riser in place.
 5. The method of claim 1 further comprising: retrieving the upper riser; retrieving and disassembling the lower riser while simultaneously expelling the buoyancy fluid from the second buoy; and detaching the second buoy from the lower riser.
 6. The method of claim 1 further comprising: retrieving the upper riser; disconnecting the lower riser from the wellhead; displacing the lower riser to a second wellhead location; and attaching the lower riser to the second wellhead location.
 7. The method of claim 5 further comprising providing a third stand-alone buoy disposed for receipt of the second buoy when the second buoy is detached from the lower riser.
 8. The method of claim 5 further comprising storing the second buoy in the third stand-alone buoy.
 9. The method of claim 8 wherein the third stand-alone buoy comprises a passively buoyant material and wherein the third stand-alone buoy is secured to the ocean floor by a plurality of mooring cables.
 10. The method of claim 2 wherein the first buoy is attached at substantially the top of the lower riser.
 11. The method of claim 10 wherein the first buoy is attached at the top of the lower riser.
 12. The method of claim 2 wherein the first buoy is attached to within about 100 feet of the top of the lower riser.
 13. The method of claim 10 wherein the second buoy is substantially adjacent the first buoy.
 14. The method of claim 13 wherein the second buoy is below the first buoy.
 15. The method of claim 13 wherein the second buoy further comprises a passive buoyancy element.
 16. The method of claim 13 wherein the first buoy comprises a passive buoy.
 17. The method of claim 16 wherein the first buoy is a passive buoy and wherein the first buoy is integrally attached to the lower riser.
 18. The method of claim 1 further comprising providing a landing ring wherein the landing ring is attached to the lower riser, and wherein the second buoy is adapted to seat on the landing ring by virtue of upward buoyancy.
 19. The method of claim 18 wherein the second buoy comprises a buoyant collar wherein the buoyant collar comprises: an upper surface and a lower surface and a bore extending therebetween; a keyway disposed in the collar, substantially parallel to the bore, the keyway extending from the upper surface to the lower surface.
 20. The method of claim 19 wherein the buoyancy collar is formed of a buoyant material.
 21. The method of claim 19 wherein the bore has a diameter and said keyway has a width and the bore diameter is equal to the keyway width.
 22. The method of claim 19 wherein the bore has a diameter and the keyway has a width and the bore diameter is larger than the keyway width.
 23. The method of claim 19 wherein the at least one chamber is open to the lower surface.
 24. The method of claim 19 further comprising a recess disposed in the upper surface of the buoyant collar.
 25. The method of claim 19 further comprising an orientation notch disposed in the upper surface of the buoyant collar.
 26. The method of claim 19 wherein the buoyant collar is disc shaped.
 27. The method of claim 19 wherein the buoyant collar is spherical in shape.
 28. The method of claim 19 wherein the buoyant collar is cylindrical in shape.
 29. The method of claim 19 wherein the buoyant collar is asymmetrical in shape.
 30. The method of claim 19 further comprising providing a centralizer element wherein the centralizer element is attached to the lower riser.
 31. The method of claim 30 wherein the centralizer element is a centralizer ring.
 32. The method of claim 1 wherein the buoyancy fluid comprises a gas wherein the gas is air or nitrogen.
 33. The method of claim 30 wherein the disconnect joint comprises an upper disconnect joint and a lower disconnect joint and further comprising providing a blow out preventer stack wherein the blow out preventer stack is attached to the lower riser.
 34. The method of claim 3 further comprising: wherein the marine riser system further comprises a first buoy attached to the lower riser; providing a landing ring wherein the landing ring is attached to the lower riser, and wherein the second buoy is adapted to mate with and removably attach to the landing ring so as to enable the second buoy to removably affix to the lower riser; wherein the second buoy is non-integral to the lower riser and wherein the second buoy is removably attached to the lower riser; wherein the second buoy is substantially adjacent the first buoy; wherein the first buoy comprises a passive buoy; wherein the first buoy is attached at substantially the top of the lower riser retrieving the upper riser; wherein the disconnect joint comprises an upper disconnect joint and a lower disconnect joint and further comprising providing a blow out preventer stack wherein the blow out preventer stack is attached to the lower riser; detaching the second buoy from the lower riser; providing a third stand-alone buoy disposed for receipt of the second buoy when the second buoy is detached from the lower riser wherein the third stand-alone buoy comprises a passively buoyant material and wherein stand-alone buoy is secured to the ocean floor by a plurality of mooring cables; retrieving and disassembling the lower riser; storing the second buoy in the third stand-alone buoy; wherein the second buoy comprises a buoyant collar that is formed of buoyant material wherein the buoyant collar comprises: an upper surface and a lower surface and a bore extending therebetween; and a keyway disposed in the collar, substantially parallel to the bore, the keyway extending from the upper surface to the lower surface.
 35. The method of claim 1 wherein the lower riser and any other devices connected thereto are together substantially neutrally buoyant before the step of disconnecting the upper riser from the lower riser so as to prevent the lower riser from forcefully rising up upon disconnecting the upper riser from the lower riser.
 36. A buoyancy tensioning system for a marine riser, said buoyancy system comprising: a riser string defined by a first end and a second end; a disconnect joint disposed in said riser string; a blow out preventer stack attached to said second end of said riser string; a first buoy attached to said riser string adjacent to said disconnect joint; a landing ring attached to said riser string between said first buoy and said second end; and a second buoy seated on said landing ring is seated, said second buoy comprising at least one chamber for receipt of a buoyancy fluid.
 37. The system of claim 36, further comprising a third stand alone buoy disposed for receipt of said second buoy when said second buoy is not mounted on by said landing ring.
 38. The system of claim 36, further comprising a centralizer mounted on said riser string between said landing ring and said second end of said riser string.
 39. A buoy system for an oil and gas marine riser string, said buoy system comprising: a first housing comprised of buoyant material, said first housing defined by a top wall, a bottom wall and a side wall joining said top and bottom walls, said walls defining an interior compartment within said housing, wherein said top wall is provided with an aperture and an opening is provided in at least a side wall or the bottom wall; and a second housing comprised of buoyant material, said second housing sized to pass through said opening in a wall of said first housing, said second housing having an aperture passing axially therethrough and at least one chamber for receipt of a buoyancy fluid.
 40. A buoyancy system for use with an offshore oil and gas riser string, said buoyancy system comprising: a buoyant collar comprising: an upper surface and a lower surface and a bore extending therebetween; a keyway disposed in said collar, substantially parallel to said bore, said keyway extending from said upper surface to said lower surface; and at least one chamber into which buoyancy fluid can be pumped.
 41. The system of claim 40 wherein said buoyant collar is formed of buoyant material.
 42. The system of claim 40 wherein said bore has a diameter and said keyway has a width and said bore diameter is equal to said keyway width.
 43. The system of claim 40 wherein said bore has a diameter and said keyway has a width and said bore diameter is larger than said keyway width.
 44. The system of claim 40 wherein said chamber is open to said lower surface.
 45. The system of claim 40 further comprising a seat disposed in the top surface of said collar.
 46. The system of claim 40 further comprising an orientation notch disposed in the surface of said collar.
 47. The system of claim 40 wherein said buoyant collar is disc shaped.
 48. The system of claim 40 wherein said buoyant collar is spherical in shape.
 49. The system of claim 40 wherein said buoyant collar is cylindrical in shape.
 50. The system of claim 40 wherein said buoyant collar is asymmetrical in shape.
 51. A method for installing an offshore oil and gas riser string between a rig at the surface of the ocean and a wellhead adjacent the ocean floor, said method comprising the steps of: securing a first buoy having a second buoy removably disposed therein adjacent the ocean floor; attaching a blowout preventer to the lower end of a riser string; attaching a landing ring on said riser string above said blowout preventer; attaching a third buoy on said riser string above said landing ring; attaching a riser disconnect joint in said riser string above said third buoy; attaching riser joints above said riser disconnect joint; positioning said riser string adjacent said first buoy; moving said riser string so as to engage the second buoy; moving said riser string with the engaged second buoy away from said first buoy; and attaching said blowout preventer adjacent a wellhead on the ocean floor.
 52. The system of claim 51 wherein the step of moving the riser string to engage the second buoy comprises passing the riser string through a slot in the first buoy, axially aligning said riser string with a bore defined in said first buoy and causing said riser string to move axially downward until said second buoy seats against said landing ring.
 53. A method for moving an offshore oil and gas rig attached to a riser extending down to the ocean floor and attached to a blowout preventer, said method comprising the steps of: providing a riser with a first buoy secured adjacent a disconnect joint and a second buoy secured to said riser below said first buoy, wherein said riser system has an upper riser portion above said disconnect joint and a lower riser portion below said disconnect joint; pumping a fluid into said second buoy so as to increase the buoyancy of said second buoy and thereby the tension on the lower riser portion of said riser string; and disconnecting said rig and upper riser portion from said lower riser portion at said disconnect joint.
 54. A passive buoyancy system for attaching to and providing buoyancy for a marine riser comprising: a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser; a plurality of stackable elements stacked on the support mandrel; wherein each stackable element comprises a buoyant material; and wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto the support mandrel to affix to the support mandrel.
 55. The passive buoyancy system of claim 54 wherein each stackable element has an upper surface and a lower surface and wherein each upper surface comprises a raised section for interfacing and interlocking with an adjacent disc.
 56. The passive buoyancy system of claim 55 wherein each stackable element comprises a mesh netting having a buoyant material within the mesh netting.
 57. The passive buoyancy system of claim 55 wherein the plurality of stackable elements comprise: a plurality of clockwise stackable elements; a plurality of counterclockwise stackable elements.
 58. The passive buoyancy system of claim 57 wherein each of the clockwise stackable elements are adapted to mate with each of the counterclockwise stackable elements when each of the clockwise stackable elements is stacked adjacent to each of the counterclockwise stackable elements and when each of the clockwise stackable elements is rotated to mate and lock with each of the counterclockwise stackable elements.
 59. The passive buoyancy system of claim 55 wherein each stackable disc further comprises a plurality of lifting pockets for receiving a lifting arm for manipulating and orientating each stackable disc onto the support mandrel.
 60. A disc handling device for assembling a passive buoyancy system for integration of the passive buoyancy system into a marine riser a frame; and a plurality of lifting arms attached to the frame with a means for interfacing with, retrieving, lifting, and rotating a plurality of stackable elements from a transport spool and a means for rotating and lowering the stackable elements onto a support mandrel.
 61. A method for assembling a passive buoyancy system for integration of the passive buoyancy system into a marine riser comprising: providing a disc handling device wherein the disc handling device comprises a frame, and a plurality of lifting arms attached to the frame with a means for interfacing with, retrieving, lifting, and rotating a plurality of stackable elements from a support spool and a means for rotating and lowering the stackable elements onto a support mandrel; providing a transport spool; providing a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser; providing a plurality of stackable elements stacked on the transport spool wherein each stackable element comprises a buoyant material wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto a shaft wherein each stackable element comprises an orientation element for interfacing and locking with an adjacently stacked stackable element; interfacing each stackable element with the plurality of lifting arms; lifting each stackable element a distance apart from each adjacently stacked stackable element; rotating each stackable element so as to configure each keyway into alignment with one another; sliding the support mandrel through the aligned keyways of each stackable element so as to dispose the support mandrel substantially within the bore of each stackable element; rotating each stackable element to an angle that allows the orientation notch of each stackable disc to interface and lock with each adjacently stacked stackable element; and lowering and loading each stackable element onto the support mandrel.
 62. The method of claim 61 further comprising integrally attaching the lower end of the support mandrel to the marine riser.
 63. The method of claim 62 further comprising integrally attaching the upper end of the support mandrel to a disconnect joint.
 64. A method of assembling a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor, the method comprising the steps of: providing a marine riser system comprising: a riser comprising an upper riser and a lower riser, a disconnect joint removably attaching the upper riser to the lower riser, and a hang-off ring attached to the lower riser; providing a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is moored to the seabed by one or more mooring lines wherein the second buoy is configured to mate with the hang-off ring of the lower riser; mating the marine riser system to the second buoy via the hang-off ring; introducing a buoyancy fluid into the at least one chamber of the second buoy; and disconnecting the upper riser from the lower riser at the disconnect joint.
 65. The method of claim 64 further comprising a spring buoy attached to at least one of the mooring lines. 